Generally, non-reciprocal circuit elements such as an isolator and a circulator are mounted on circuit boards of microwave devices such as a microwave amplifier and a microwave oscillator. In such non-reciprocal circuit elements, a magnetic field in one direction is provided by a permanent magnet into a magnetic body of, e.g., ferrite for causing microwaves to magnetically resonate, a central conductor including I/O terminals is arranged at a surface of the magnetic body, and a transmission path for microwaves input from one of the I/O terminals is rotationally changed to another one according to the right-handed screw rule invoked by a gyromagnetic effect.
With the recent reduction in size and weight of circuit boards of microwave devices as described above, there is a need for development of a small thin one which has a simpler structure and superiority in assembly compared with a conventional one and can be directly placed and mounted on a circuit board, among non-reciprocal circuit elements of this type.
Under the circumstances, the present inventors have proposed, in Patent Literature 1 below, a non-reciprocal circuit element including a central conductor with radially branched I/O terminals, upper and lower ferrite plates facing each other with the central conductor therebetween, and upper and lower conductor plates facing each other with the upper and lower ferrite plates therebetween and having a magnet arranged on an upper surface of the upper conductor plate which applies a magnetic field to the upper and lower ferrite plates in a fixed direction, wherein hole portions are provided at positions, facing the I/O terminals of the central conductor, of the lower ferrite plate, notches are formed at positions, facing the hole portions, of the lower conductor plate, the I/O terminals of the central conductor are inserted in the hole portions, drawn to the rear side, and bent, and the upper and lower conductor plates are connected to each other by side walls to form a closed magnetic circuit.
According to the non-reciprocal circuit element with the above-described configuration, the I/O terminals of the central conductor are substantially flush with a rear surface of the lower conductor plate, which allows the element to be directly placed and surface-mounted on a circuit board. Additionally, the non-reciprocal circuit element needs only provision of hole portions in the lower ferrite plate and is simple in configuration. The non-reciprocal circuit element has the advantages of a smaller number of components and easier assembly.